The term vacuum has two different meanings. In its strictest sense, a vacuum is a region of space completely lacking any form of matter. The term represents absolute emptiness. One problem with this definition is that it describes an ideal condition that cannot exist in the real world. No one has ever discovered a way to make a perfect vacuum of this kind.

For that reason, the term vacuum also is used to describe regions of space from which the greatest possible amount of matter has been removed. In most cases, a vacuum is a container from which all gases have been removed as completely as possible.

Actually, the closest thing to a perfect vacuum is outer space. Astronomers believe that the space between stars consists in some cases of no more than a single atom or molecule per cubic kilometer. No vacuum produced on Earth comes even close to this condition.

The usual procedure for making a vacuum is with a vacuum pump. The pump, which consists of a piston (a sliding valve) in a cylinder, is attached to a closed container. With each stroke of the pump, some of the gas in the container is removed. The longer the pump operates, the better the vacuum produced in the container. To achieve the very best vacuums, however, special types of equipment are necessary.

Vacuums have many applications in scientific research, industry, and everyday life. Perhaps the most common example of the use of a vacuum is the household vacuum cleaner. The fan in a vacuum cleaner continually removes air from a canister, creating a partial vacuum. Atmospheric pressure outside the vacuum cleaner pushes air into the canister, taking along with it dust and dirt stirred up by the brush at the front of the vacuum cleaner.

Another common application of vacuums is a thermos bottle. A thermos bottle consists of two bottles, one nested inside the other. The space between the two bottles consists of a vacuum. In the absence of air, heat does not pass between the two bottles very easily. Hot liquids inside the container retain their heat, and cold liquids stay cold because heat cannot pass into them.

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